Ground Impedance

[iggy2]

The Code generally requires a "low impedance" path to ground, via an equipment grounding conductor (which can be of many forms - a conductor, RGS, EMT, etc. per 250.118). Is there any standard on what constitutes a "low impedance"??? The only definition I see is for "Effective Ground-Fault Current Path", which uses the term 'low impedance.

I am involved in a situation where existing equipment grounding conductors (via metallic raceways) is, worst case, about 1/10 of an ohm. Other readings are lower. It has been suggested that these values are unacceptable, and a copper EGC be added to the raceways, and I am trying to judge if these values are in fact unacceptable.

The grounding electrode impedance is 2.5 ohms, so that is not in question (unless because it seems unrealistically low...)

 

[iwire]

The steel raceway has less impedance than the wire EGC you would pull inside it.

 

[peter d]

The Code generally requires a "low impedance" path to ground, via an equipment grounding conductor (which can be of many forms - a conductor, RGS, EMT, etc. per 250.118). Is there any standard on what constitutes a "low impedance"??? The only definition I see is for "Effective Ground-Fault Current Path", which uses the term 'low impedance.

I am involved in a situation where existing equipment grounding conductors (via metallic raceways) is, worst case, about 1/10 of an ohm. Other readings are lower. It has been suggested that these values are unacceptable, and a copper EGC be added to the raceways, and I am trying to judge if these values are in fact unacceptable.

The grounding electrode impedance is 2.5 ohms, so that is not in question (unless because it seems unrealistically low...)

Out of curiosity, who has determined these values are unacceptable? EMT with a separate bonding/grounding conductor is a relatively new thing. For decades EMT was used as an EGC until some hysteria broke out over that, and the practice has all but been abandoned.

 

[ActionDave]

Out of curiosity, who has determined these values are unacceptable? EMT with a separate bonding/grounding conductor is a relatively new thing. For decades EMT was used as an EGC until some hysteria broke out over that,

I agree. There are a few old buildings I work on for regular customers that do not have an EGC buss in the panels. Never been a problem for fifty years.

and the practice has all but been abandoned.

Not for me. I don't do much new construction now though. 2017 code is going to require a green in conduits that feed RTUs. I'll be curious to see if the requirement gets expands in future cycles.

 

[jap]

A low impedence ground path is needed to clear a fault.
Sounds like yours is pretty low.
I guess if someone wanted to, they could create their own controlled low amperage ground fault on the existing conduit and if the fuse blew, I'd consider is sufficient.


JAP>

 

[iggy2]

Out of curiosity, who has determined these values are unacceptable?

Well, the original design consultant (qualifications unknown to me) has stated that the grounding system is "concerning". And that was passed on to the AHJ who now feels something should be done. I was called in as a third party, but cannot find anything quantitative to say that the values are not concerning, other than to compare the values to the resistance of a copper #8 ground conductor, for example.

 

[don_resqcapt19]

Take a look at the fault clearing time for the circuit. If the existing path keeps you in the instantaneous trip part of the curve, there would be almost nothing gained by reducing the impedance of the fault clearing path.

 

[winnie]

My gut feeling/impression from reading about this stuff (rather than a standard/code that I can point you to) is that a 'low impedance' is one for which a bolted fault would put the OCPD into the 'instantaneous' or 'magnetic trip' range rather than the thermal trip range.

As iwire has noted, an intact steel raceway will have lower impedance than a reasonable EGC pulled in the raceway. IMHO the only potential concern with using the raceway as EGC is continuity. If the facility appears to have problems with keeping raceways in proper repair, then their ground system is concerning (but they have other problems).

Another question to ask: is the concern about ground impedance something to do with data or signal lines having problems? Even with good low impedance ground you might see ground loops which will cause problems on such systems if not properly designed.

-Jon

 

[iggy2]

Another question to ask: is the concern about ground impedance something to do with data or signal lines having problems?

No. The issue was raised when the EC reported arcing when touching or separating metal parts (such as taking a metal plate off). The Owner recalls no such history, and feels it could just as easily have been static. Some ground tests were done, and the original consultant stated that the report indicates "high impedance" [SIC - resistance readings were taken - not impedance readings...]. This has been passed onto the AHJ.

I don't believe it would surprise anyone that it would be quite costly and intrusive to add a ground conductor to each and every feeder and branch circuit - probably with no benefit.

 

[jap]

No. The issue was raised when the EC reported arcing when touching or separating metal parts (such as taking a metal plate off). The Owner recalls no such history, and feels it could just as easily have been static. Some ground tests were done, and the original consultant stated that the report indicates "high impedance" [SIC - resistance readings were taken - not impedance readings...]. This has been passed onto the AHJ.

I don't believe it would surprise anyone that it would be quite costly and intrusive to add a ground conductor to each and every feeder and branch circuit - probably with no benefit.

Sounds like a Neutral issue to me not a grounding issue.

JAP>

 

[winnie]

Sounds like a Neutral issue to me not a grounding issue.

I agree with jap.

If you get arcing when you open an egc, that suggests current flowing through the egc.

If you have a neutral to egc fault, then the egc becomes a parallel path for the neutral current. Some of the neutral current flows on the appropriate neutral wires, some flows on the conduit...until you break the conduit path.

Another related possibility is the use of a bootleg neutral or a 300.3 violation. If current goes out to a load via conductors in one conduit, and returns via conductors in another conduit, then that big loop can act like a transformer, inducing current flow in the conduit.

-Jon

 

[iwire]

The issue was raised when the EC reported arcing when touching or separating metal parts (such as taking a metal plate off).

I don't see how a spark from taking a metal plate off a device could be anything other than static.

 

[roger]

worst case, about 1/10 of an ohm. Other readings are lower. It has been suggested that these values are unacceptable,

:jawdrop:And that's not low enough?

Roger

 

[jap]

I don't see how a spark from taking a metal plate off a device could be anything other than static.

I'd bet Its not static.
The person taking off the lid is becoming part of the return path himself, its just that he has too much resistance feel a noticeable shock.

JAP>

 

[jap]

We were above the ceiling in a business once running pipe and you could hold onto one end of a 10' piece of pipe, scrape it along an existing conduit and throw sparks left and right without actually feeling a shock.

This issue was not resolved until a new service was built and the very old "very undersized" neutral conductor was upgraded to full size.
Once that was done , the issue went away.

If it was a static issue at the time instead of the very undersized neutral, it was resolved with the new service with the full size neutral since nothing else was changed.

JAP>

 

[iwire]

I'd bet Its not static.
The person taking off the lid is becoming part of the return path himself, its just that he has too much resistance feel a noticeable shock.

JAP>

Your body has too much impedance to draw a spark at 120 - 277 volts.

 

[jap]

Your body has too much impedance to draw a spark at 120 - 277 volts.

Then I guess it was static then.



JAP>

 

[iwire]

Then I guess it was static then.



JAP>

For what it is worth I am not saying that about the problem you described with the conduits.

My comments have only to do with the OPs description of removing a plate.

 

[jap]

For what it is worth I am not saying that about the problem you described with the conduits.

My comments have only to do with the OPs description of removing a plate.

In our case any metal object that we had a hold of would create not a single spark but a shower of sparks if you were to rub it along the existing conduits.
Much like the OP's reference to a guy taking off the metal cover where it rubs the box when your taking it off.

I dont know the technical reasoning for it I'm just so redneck, I thought it was cool and actually played with it for several minutes to see what all I could make it do.

JAP>

 

[jap]

Much like the wheels on the tracks of the old time electric train sets.

JAP>